1. Field of the Invention
The field of the invention is that of the transmission of pictures in a digital channel.
The invention relates notably to the transmission of digital television signals, for example by radio link, where said signals can be received, on the one hand, by moving receivers, especially in difficult conditions of reception and, on the other hand, by fixed receivers requiring high picture quality.
The invention can be applied to any type of picture transmission, such as that of so-called standard television, corresponding to a picture format of the order of 720 by 575 pixels, high-definition television (HDTV) or visio telephony.
With the huge increase in the number of digital channels and their transparence to the data transmitted, the use of these channels for picture transmission is likely to increase substantially in the years to come.
However, this transmission of pictures on digital channels is not a trivial matter. For, the transmission bit rate of the different existing channels does not enable the transmission of moving pictures as such in real time.
2. Description of the Prior Art
There are many known bit rate reduction or encoding techniques. These techniques generally comprise three steps:
A step for representing the picture as compactly as possible by using the redundancies that exist in the picture or in the sequence of pictures;
A step for the quantization of this new representation;
A step for the assignment, to each quantized value, of a code word that will be transmitted and will enable the decoder to reconstruct the sequence of transmitted pictures.
These known encoding methods generally make it possible to obtain bit rates that enable the transmission of high quality pictures to fixed receivers. By contrast, the transmission channels and encoding methods used do not enable reliable reception in moving receivers, especially in urban conditions, i.e. in conditions of multiple propagation (Rayleigh process) generating phenomena of fading, and in the presence of parasitic noise and jamming.
Broadcasting to moving receivers therefore calls for the use of more reliable techniques of redundancy encoding and/or modulation, to the detriment of the number of pieces of information, hence to the detriment of the definition of the transmitted picture.
It is known, moreover, that the bit rate of the transmission channels is fixed while the data bit rate, following the encoding, is most usually variable. This requires the presence of a buffer memory element and regulation means that act on the encoding and, for example, on the quantization levels as a function of the rate with which this buffer memory is filled.
These regulation means have the drawback of acting without distinction on any coefficient to be transmitted, whereas it is known that some of these coefficients are more important than others, notably from the psychovisual viewpoint. Thus a coefficient that is less important as regards visual acuity, for example because it corresponds to a piece of information concerning a high spatial frequency, may be transmitted with high encoding definition if the buffer memory is scantily filled, while an essential coefficient of the same picture, arriving at a time when the buffer memory is saturated, will be encoded with a lower definition. In other words, information of little importance would then have been encoded normally, while information of greater relevance would have been under-coded.
The invention is designed to overcome these different drawbacks.